The present invention relates to a tone control device for an audio equipment.
One of the channels of a graphic equalizer, which is a tone control device, can be provided by using a circuit which satisfies the following transmission function T.sub.(s) : ##EQU1## where S is the Laplace operator, .omega..sub.o is center angular frequency, Q is sharpness, K.sub.1 is attenuation factor at the angular frequency .omega..sub.o and K.sub.2 is amplification factor at the angular frequency .omega..sub.o.
By commonly dividing the numerator and the dominator of the right hand term of equation (1) by S.sup.2 and then multiplying the results of the divisions with a certain common conditional parameters, respectively, the following equation (2) is obtained. ##EQU2##
The numerator and the dominator of the right hand term of the equation (2) are divided by a common constant ##EQU3## respectively. The result is as follows: ##EQU4## where B.sub.(s) is a transmissions function of a bandpass filter, which can be represented as follows: ##EQU5## Since T.sub.(s) =e.sub.o /e.sub.i, where e.sub.i and e.sub.o are an input voltage and an output voltage of the channel, the followings are obtained: EQU x=e.sub.i -xB.sub.(s) K.sub.1 EQU e.sub.o =x+xB.sub.(s) K.sub.2 ( 5)
FIG. 1 shows a block diagram of a circuit which satisfies the equations (5) and FIG. 2 shows an example of concrete circuits embodying the block circuit in FIG. 1, in which a reference numeral 10 shows a band pass filter having transmission function B.sub.(s) and 13 and 14 are amplifiers.
FIG. 2 shows an example of the conventional tone control device which comprises a plurality (n) of parallel connected bandpass filters each constituted as shown in FIG. 2 to boost or cut the grains thereof at a corresponding number (n) of respective specific frequencies. In FIG. 2, outputs of the bandpass filter 10.sub.1 to 10.sub.n are connected through variable resistors 12.sub.1 to 12.sub.n to the amplifiers 13 and 14, respectively. In this conventional tone control device, there is a problem of degradation of signal quality. This problem becomes more severe when the Q values and the center angular frequencies are to be variable.